This invention relates to methods and apparatus for mixing strata of material, and more particularly to methods and apparatus for mixing layers of solid and/or semi-solid materials on the surface of the earth to convert certain materials, such as waste byproducts, into more useful materials, such as fill material for roadbeds and the like.
A specific example of a material which is generally regarded as being unusable, but which could potentially be mixed with another material to form a useful fill material, is phosphogypsum. Phosphogypsum is a byproduct from the production of phosphoric acid, which is used in a dehydrated form as fertilizer. Phosphate rock mined from the earth contains a small amount of radionuclides. After the phosphoric acid is produced from the ore, the byproduct phosphogypsum has a slightly higher concentration of radionuclides than the unprocessed ore. The EPA restricts the use of materials containing more than 10 picocuries per gram. Most of the nearly 1 billion tons of byproduct phosphogypsum stocked piled in Florida and adjoining states averages 30 picocuries per gram.
The Florida Institute of Phosphate Research (FIPR) has, for many years, been attempting to develop environmentally acceptable ways of utilizing the huge stock piles of phosphogypsum byproducts for economic benefit. A proposed use for phosphogypsum byproduct which is expected to be deemed environmentally acceptable involves mixing phosphogypsum byproducts with another earthen material, such as soil, sand, stone, clay, loam, and/or other byproduct materials such slag, dross, cinder, and the like, or combinations thereof. Such materials can be mixed to form aggregates which are environmentally acceptable and which exhibit suitable properties, such as good packing, percolation and the like, for use as a fill material for roadbeds, airport runways, parking lots, sound abatement berms, and earthen levees.
Although mixtures or aggregates of earthen materials and waste materials could potentially be usefully employed as fill for roadbeds and the like, thus converting the large volumes of unused materials into valuable construction materials and concomitantly freeing large areas of land on which such waste materials are stock piled for more valuable and/or productive uses, a major impediment to implementation of such beneficial practices is the apparent difficulty in economically forming such aggregates. The use of conventional mixing equipment for forming bulk aggregates, such as equipment typically used for preparing concrete and cement, would be prohibitively expensive. The use of conventional earth moving equipment, such bulldozers, excavators and the like, can mix bulk quantities of waste materials with earthen materials would also be impractical because of the difficulty, time and expense associated with achieving sufficiently thorough mixing.
Known equipment for in situ mixing of strata of material are generally only capable of reaching a depth of about two feet. Although such known equipment may be successfully employed for mixing strata of material to convert undesirable mineral byproducts into useful construction fill materials, even greater economic and environmental benefits could be attained with methods and apparatus for in situ mixing of material strata to a greater depth.